The invention relates to a protective device for a neutron tube comprising an ion source whose anode is brought to a positive potential relative to the cathode by means of a source supply, and whose accelerated ion beam strikes a target disposed on an insulating support and brought to a negative potential supplied by a high tension (HT) supply, said protective device being composed of electric elements limiting the tube current and/or the target voltage.
In their usual application neutron tubes operate under conditions compatible with the ability to dissipate heat, particularly from the target and its support.
Their specifications are in addition designed for wide-dynamic operating modes:
continuous mode operation necessitates dimensioning dictated by the insulation; PA0 rapid recurrence, pulsating mode operation leads to extraction structures permitting relatively high ionic currents. PA0 increasing the tube current by means of an increase of the discharge current of the ion source under arc operating conditions, with a higher anode voltage; PA0 a possible increase of the target voltage by a factor close to 1.5. PA0 a screen printed resistor disposed helically on the outer face of an insulating cylinder serving as a support for the target; PA0 an insulated resistive wire (high temperature technology) wound turn-to-turn or in the form of a pancake coil; PA0 high tension resistive elements in series, disposed inside an envelope of alumina (or any other insulating material compatible with tube technology and VHT requirements), which may either be in communication with the tube or in a gas atmosphere depending on the technology of the resistive elements (temperature characteristics, degassing), the method of connection and assembly (electric field in resistors and connection wires), and the maximum level of voltage drop accepted in the resistor by the manufacturer.
The ion sources themselves, which are often of the Penning type, are provided with extraction apertures of fairly large dimensions in order to make it possible to obtain a high extraction yield with a low operating pressure. Furthermore, they can operate under arc type discharge conditions with pressure values which are still compatible with operation of the tube, and the very high voltages which can be applied to these tubes may make it possible to extract large ionic currents during short periods of time.
All these considerations show that, particularly for tubes of small dimensions, it is possible to use a neutron tube considerably beyond its nominal use, for example for utilization with an intense pulsating neutron flux. This can be achieved by:
The electrical limiters usually installed in the supply circuits of the ion source anode and of the target for the purpose of protecting the supply and the tube can be dispensed with and replaced by new elements suitable for a new use.